In the conventional method for forming an ohmic electrode of a compound semiconductor such as an n-type GaAs, GaAlAs, etc., in order to form the product in a desirable form on the semiconductor substrate, a photo resist pattern is formed on the n-type layer surface in many cases, and vacuum evaporation is performed by a resistance heating method with an alloy of Au and Ge as a vacuum evaporation source, and Ni is vacuum-deposited by a resistance heating method or by an electron beam deposition method. Namely, as shown in FIG. 3, the pattern 3 is formed (FIG. 3 (b)) using a photo resist on n-GaAlAs 2 formed on p-GaAlAs 1. Then, the electrode materials Au/Ge layer 4 and a Ni layer 5 are vacuum-deposited on the entire surface (FIG. 3 (c)), unnecessary portions are removed by a lift-off method, and the electrode pattern 6 is formed as shown in FIG. 3 (d). In order to obtain an ohmic property after electrode materials are vacuum-deposited, alloying treatment is performed by heating at 400.degree. to 500.degree. C. in a nitrogen or hydrogen atmosphere.
A composition profile to a depth from the electrode surface was obtained on such a conventional type Au-Ge/Ni electrode using SIMS (secondary ion mass spectrometry), the results being shown in FIG. 4. In the figure, the abscissa represents the depth from the electrode surface (in any unit), and the ordinate is the component strength (in any unit).
In the figure, the depth with equal strength of Au and Ga approximately corresponds to the interface between the electrode and semiconductor, and the Ni content at this position in as high as 2.38 wt %. Natural oxide film is present on the AlGaAs surface, and Au-Ge solution are aggregated on the portion without an oxide film, developing the irregular alloying, the so-called ball-up phenomenon. Because Ni shows strong solid phase reaction with GaAs, a part of the Ni is diffused to the AlGaAs interface. Thus, GaAs is decomposed in solid phase, and the compounds such as NiAs, .beta.-AuGa, etc. are formed. By this solid phase reaction, a natural oxide film on the AlGaAs semiconductor surface is removed, and this prevents ball-up phenomenon. In an Au-Ge/Ni electrode by the conventional method, Ni is diffused in large quantity to the AlGaAs layer as is evident from FIG. 4, to prevent ball-up, such that it is necessary to vacuum-deposit Ni in a large quantity. Also, Al is piled up on the electrode layer surface. Because Al is easily oxidized and is turned to insulating material when oxidized, contact resistance is increased, and the ohmic property is lost in some cases.
In the conventional method, when the mixed crystal ratio x of Al in an n-type Ga.sub.1-x Al.sub.x As (0.ltoreq..times.1) or an n-type Ga.sub.1-x Al.sub.x As with low carrier density is 0.5 or more, contact resistance is not sufficiently low. If alloy temperature is set at higher value in order to decrease contact resistance, ball-up phenomenon occurs. Irregularities are found on the surface of vacuum-deposited metal, and detachment occurs during wire bonding.